Interplay between Oxygen Octahedral Rotation and Deformation in the Acentric ARTiO4 Series toward Negative Thermal Expansion
Interplay between Oxygen Octahedral Rotation and Deformation in the Acentric ARTiO4 Series toward Negative Thermal Expansion
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无心 ARTiO4 系列中氧八面体旋转与负热膨胀变形之间的相互作用
DOI：
10.1021/acs.chemmater.2c01245
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发表时间：
2022
影响因子：
8.6
通讯作者：
Fujita Koji
中科院分区：
文献类型：
--
作者：
Yoshida Suguru;Akamatsu Hirofumi;Gibbs Alexandra S.;Kawaguchi Shogo;Gopalan Venkatraman;Tanaka Katsuhisa;Fujita Koji
众所周知，钙钛矿相关氧化物中氧配位八面体的尺寸、形状和连通性对其材料性能的工程设计起着关键作用，此外，不同类型氧八面体畸变之间的相互作用可以为结构驱动的功能控制开辟新的策略。本文中，我们报道了层状钙钛矿agtio4 （R：稀土）的双轴负热膨胀（NTE）是由改变八面体连接和形状的结构扭曲之间的相互作用引起的，这与传统的NTE材料中的机制根本不同。先前认为agtio4具有正交（Pbcm）结构，但我们的实验和理论研究表明，由于（Φ00）（0Φ0）型tio6八面体旋转，该化合物采用了偏心四方（P4 ~ 21m）结构，正如之前报道的Na和K类似物nartio4和KRTiO4一样。深入的结构分析表明，八面体旋转与八面体变形的竞争驱动了双轴NTE；加热引起的旋转幅度减小导致tio6八面体发生八面体变形，即面外伸长和面内压缩，导致晶格参数saandb收缩（a=b）。Ag+-R3+层序产生了一个内置的电场，迫使Ti4+偏离中心，这是八面体旋转和变形共存的不利来源。尽管两种八面体扭曲之间的竞争被预测在artio4系列的其他成员（A= Na， K和Rb）中也很活跃，但我们没有在实验中观察到Na成员的双轴NTE。对计算电子结构的详细分析强调了Ag-O-Ti共价键在增强agtio4的八面体变形中所起的重要作用，这是导致双轴NTE的直接原因。本研究提供了一个重要的例子，从八面体框架的不同扭曲之间的耦合出现功能性质。
Tailoring size, shape, and connectivity of oxygen coordination octahedra in perovskite-related oxides is known to play a key role in engineering their material properties, and furthermore, the interplay among the different types of oxygen octahedral distortions can open up new strategies for structure-driven control of functionalities. Here, we report that in layered perovskites AgRTiO4(R: rare earth), the biaxial negative thermal expansion (NTE) arises from the interplay between the structural distortions that alter the octahedral connectivity and shape, which is a fundamentally different mechanism from those in the conventional NTE materials. AgRTiO4was previously identified as having an orthorhombic (Pbcm) structure, but our experimental and theoretical study reveals that this compound adopts an acentric tetragonal (P4̅21m) structure due to (Φ00)(0Φ0)-type TiO6octahedral rotations, as in the previously reported Na and K analogs, NaRTiO4and KRTiO4. Thorough structural analysis reveals that the competition of the octahedral rotations with octahedral deformations drives the biaxial NTE; the decrease in the rotation amplitude caused by heating results in octahedral deformations, i.e., an out-of-plane elongation and an in-plane compression of TiO6octahedra, leading to shrinkage of the lattice parametersaandb(a=b). The Ag+-R3+layered ordering produces a built-in electric field compelling Ti4+to off center, which is the source for the otherwise unfavorable coexistence of octahedral rotations and deformations. Despite the competition between the two octahedral distortions being predicted to be active in other members of theARTiO4series (A= Na, K, and Rb) as well, we do not observe experimentally the biaxial NTE for the Na members. Detailed analysis of calculated electronic structures highlights the essential role played by Ag–O–Ti covalent bonding in enhancing the octahedral deformation of AgRTiO4, which is directly responsible for the biaxial NTE. The present study provides an important example of functional properties that emerge from the coupling among distinct distortions of octahedral frameworks.